One may be placed on a beach to monitor risk, according to Spooner (4,5)
I believe the answer is:
deck chair
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for deck chair that I've seen before include "Seaside seat" , "Folding seat on board" , "Portable folding seat as for ship" , "Folding canvas seat" , "Seat used on beaches" .)